1. Field of the Invention
The present invention relates to a method and apparatus for converting interlaced raster scan video inputs to a non-interlaced sequential output for driving cathode ray tubes, a three gun projector, liquid crystal displays or any other sequential input display device.
2. Discussion of Prior Art
Conventional television signals are generally conveyed in either NTSC, PAL, or SECAM formats. Each of these formats utilizes a raster scan in which the electron beam "paints" a picture by causing a number of lines of pixel elements to phosphoresce when stimulated by the electron beam in the cathode ray tube. For discussion of various problems in prior art devices NTSC black and white television video will used with its 30 Hz and 60 Hz frame and field rates, respectively.
During scanning, in order to reduce signal bandwith required to transmit or store television images, each of the above videos utilizes "interlacing". Interlacing is a term applied to image scanning in which, during one field or sequence of lines painted by the electron beam, the even numbered lines would be painted and subsequently, in the next field, the odd numbered lines would be painted. The fields are generally provided at a 60 Hz field rate (so as to provide a 30 Hz frame rate with each frame comprising two fields) and the "persistence" of the individual phosphors is such that, when stimulated to emit light, the human eye can see no difference between the fields and "perceives" the picture to be flicker free even at the provided 30 Hz frame rate.
FIGS. 1A-1C illustrate a typical video raster scan input and its operation. FIGS. 1A-1C numbers the lines being painted as well as the pixel number being painted along the vertical and horizontal axes, respectively. In FIG. 1A which is identified as T1 indicating the first field, the odd numbered lines are scanned and for this particular input image (a diagonal line from the upper left to the lower right), pixels 1 and 3 are energized in lines 1 and 3 respectively. In the second frame shown in the second field as shown in FIG. 1B as T2, the even numbered lines are scanned with a result that pixels 2 and 4 are energized in lines 2 and 4. The result due to the persistence of the phosphors on a television screen is the "averaged" image shown in FIG. 1C which results in the desired diagonal line image.
Computers which provide serial non-interlaced outputs, are used to drive various display devices such as three gun projectors, liquid crystal display devices as well as cathode ray tubes at picture rates of 50-80 Hz. Increasingly it has been desirable to be able to combine television video inputs with computer generated video inputs to drive such non-interlaced devices or sequential input devices, which, in Europe, are called progressive scan devices and include matrix devices (such as LCD display panels) which have been manufactured for sequential address operation. Quite obviously, if every other line is not supplied, the picture quality will be seriously degraded.
Several attempts have been made in the past to directly drive non-interlaced devices with conventional video inputs, most often by deleting one of the two fields utilized in making up the frame to be displayed. This is illustrated in FIGS. 2A-2C which shows that the second field, shown in FIG. 2B as T2, contains no data and thus only the scanning of the odd numbered lines (1, 3, 5, etc.) comprises the video data displayed. As a result, the diagonal line which is displayed as shown in FIG. 2C will be relatively weak or diluted because it is missing the even numbered lines and pixel information.
FIGS. 1A through FIG. 8 in this application illustrate single pixels in a "black and white" representation have been shown. Differences in cross hatching represent pixels energized in one or the other of the two fields illustrated and, where there are two lines of cross hatching in single pixel, that indicates that the pixel has been illuminated or energized in both fields of the frame illustrated. Quite clearly, energization in a black and white representation could be either black or white or any one of a number of shades of gray in between. However, it should also be understood that although black and white has been chosen for simplicity of understanding of applicant's invention, the same problems and same advantages appear with respect to color although due to the requirement of providing red, green and blue output energization levels, the pixel represented by a dot of color would actually be three sub-pixels, each representing one of the primary colors.
Another attempted solution to the problem of converting interlaced raster scan video to a sequential video for driving non-interlaced devices is to utilize a dual port frame buffer memory system. A characteristic of such a dual port system is that it can enter and store data in the system at the same time data is being read out of the system, hence the name "dual port" memory. However, the storage requirements of such a memory are rather substantial. Many output devices have a field 640 pixels wide.times.480 lines high, which, if color is provided, each pixel is comprised of three subpixels. Both fields T1 and T2 would be read into such a memory so that it contains both the odd numbered lines and the even numbered lines at the same time. Then, the data is sequentially read out for line 1, line 2, line 3, line 4, etc. The memory requirements of the above is quite extensive and expensive.
Video, when displayed by a high resolution display device, has particular problems in displaying video motion. It is known that a 30 Hz frame rate is about at the limit of perceived flicker for the human eye. Where the leading edge of an image in motion across the screen moves, the pixels at the leading edge change at a 30 Hz rate for those devices which store both even and odd interlaced fields in a frame store memory. This 30 Hz change or flicker at the leading edge of a moving image can be very annoying to the viewer especially in a high resolution output device because it occurs at a 30 Hz rate. This phenomenon has been called "dynamic jaggies" by those in the field and is a significant problem in displaying video on n high resolution displays.
In view of the above, there is a need for an inexpensive effective manner of converting raster scan interlaced video to a non-interlaced format for driving video display devices while at the same time reducing perceived flicker and maintaining image resolution.